SECTION II. DISCOSTOMATA-SARCOCRYPTA. 365 



founder of the future colony, PI. X. Fig. 26, was scarcely to be distinguished from 

 an ordinary representative of the genus Oikomonas, it being entirely naked, attached 

 by a prolongation of its posterior extremity, and possessing merely a single terminal 

 flagellum. The collar being next developed, the animalcule for a while was 

 indistinguishable from such a member of the genus Monosiga as M. Stcinii, while 

 finally, a thin mucilaginous film being thrown out around its body, see Fig. 27, the 

 appearance presented was that of an early condition of Salpingmca ampulla, as 

 dehneated at PI. III. Figs. 19 and 20. From this stage onwards, two or more 

 zooids being now included within the mucilaginous matrix, as shown at Figs. 24 and 

 25, the characteristic aspect of the genus Protospongia as here defined is permanently 

 assumed. In many of the smaller colony-stocks, as illustrated by Figs. 21 and 23, 

 clear traces remain of their derivation through the quadruple plan of segmentation 

 of a single primary unit. 



By far the most interesting point connected with the structural and developmental 

 features of this type remains to be discussed. As previously intimated, it is, so 

 far as known, the nearest concatenating form between the respective groups of 

 the ordinary Choano - Flagellata and the Spongida. Furthermore, it may be 

 consistently accepted as furnishing a stock-form from which by the process of 

 evolution all sponges were primarily derived. A comparison of the figures illustrative 

 of this species with those included in this treatise relating to the organography of 

 the class Spongida, is alone needed to make clear this postulate. On making such 

 comparison it will be at once recognized that typical colony-stocks of Protospongia, 

 as shown at PI. X. Figs. 20, 21, 22, correspond in a most remarkable manner 

 with a fragment of the mucilaginous cytoblastema, with its incorporated collared 

 monads, amcebiform cytoblasts, and sporular elements of any ordinary sponge-stock, 

 and more especially with that of such a non-spiculiferous type as Halisarca Dujardinii 

 or H. lobularis. It needs indeed but a slight modification of the disposition of the 

 zooids of Protospongia, to such an extent that in place of protruding on the external 

 surface of the mucilaginous zoocytium, they should debouch upon saccular invagina- 

 tions of this matrix, to produce what would have to be accepted as an undoubted 

 though very rudimentary sponge-stock. The establishment of free intercommunica- 

 tion between these saccular monad aggregates through the means of tubular canals, 

 is alone wanted to further transform such a sponge-stock into a typical representative 

 of the genus Halisarca. 



In all minor structural and developmental details the zooids of the Protospongia 

 Haeckeli accord essentially with the simpler, naked, Choano-Flagellata previously 

 described ; but in their extreme plasticity, in their excretion and occupation of a 

 common gelatinous matrix, and in the retention of the more ordinary reproduc- 

 tive products within this matrix, this specific type unmistakably manifests its near 

 affinity to the group of the Spongida. 



Sectio7i II. DISCOSTOMATA-SARCOCRYPTA 

 {or SPONGIDA). 



Collared monads structurally resembling those of the Discostomata- 

 Gymnozoida, but hidden or immersed within variously modified inter- 

 communicating chambers of a common gelatinous matrix or cytoblastema, 

 which may or may not be strengthened by supplementary skeletal ele- 

 ments. 



The necessity of accepting the sponges as peculiarly modified colony-stocks 

 of collared flagellate Infusoria, which correspond in every essential detail with 

 the simpler or independent types previously described, is abundantly demonstrated 

 in Chapter V., devoted to that special group of organisms, and is indeed self- 

 evident on examination and comparison of the plates in this volume numbered II. 

 to VI. and VII. to X. devoted respectively to the organization of the sponges and 



